Sella-Nasion line revisited

The anterior cranial base or the Sella-Nasion (SN) line is often used by orthodontists as a reference line for assessment of dentofacial deformities. Its most important value is its relative stability, practicality and the ease of location of both points Sella and Nasion. Reliability of the SN line as a suitable assessor for exact facial measurements has been discussed before. The intent of this study was to identify unusual SN rotations for subjects acquiring normal and abnormal skeletal profiles and hence correct values associated with SN line, both in vertical and horizontal planes. A random sample of 150 British white school children aged 9-12 years old was selected. Lateral head films of the children were traced and digitized. Point Nasion was joined to posterior nasal spine and also to point Sella, thus producing a Z angle when the posterior nasal spine is extended to anterior nasal spine. Means and standard deviations were calculated. The mean angle S-N-PNS was 38 degrees +/- 5 while the mean of angle N-PNS-ANS was 47 degrees +/- 3. Comparison of these angles for patients of the same age group with the mean Z angle can identify rotation of the palatal and Sella-Nasion planes. Hence corrections of angular measurements involving Sella point can be performed.